Electrochemical Characteristics of Densely-Packed Li(Ni1/3Co1/3Mn1/3)O2 Single Crystals As an Additive-Free Electrodes for Advanced Lithium-Ion Secondary Battery

Abstract

Introduction Lithium ion rechargeable batteries (LIBs) have been used in many applications, including portable devices and electric vehicles (EVs). The improvement of volumetric capacity and cycle life are strongly required for extending their operating time and traveling distance. Electrodes in commercially available LIBs include an active material with a conductive agent and a binder. Since the volumetric ratio of the active material and others is approximately 50:50, an increase in the packing density of the active material is directly linked to the improvement of the energy density. Ultimately, it can be said that electrode consisting of only the active materials is desirable for the achievement of higher energy density LIBs. In this study, we demonstrate the preparation and the electrochemical characterization of densely-packed Li(Ni1/3Co1/3Mn1/3)O2 (NCM) single crystal electrodes, directly deposited on current collector substrate by using flux growth approaches. Experimental The flux-method, which is one of the liquid-phase method, was used for the growth of NCM crystals on Pt substrate via a heterogeneous nucleation reaction. Regent-grade LiNO3, Ni(NO3)·6H2O, Co(NO3)·6H2O, and Mn(NO3)·6H2O were used as raw materials and Li2MoO4 was used as a flux. A designated amount of Li2MoO4 was added to the stoichiometric amount of raw materials of NCM. After dry mixing, the mixture was put on the Pt substrate (φ = 14 mm) placed on the bottom of alumina crucible. Then, the crucibles were heated to 900 or 1000 °C for 10 h. The NCM/Pt electrode substrate was washed with warm water for the separation with the remaining flux. The shape and structure of the crystal layers were characterized by scanning electron microscopy (SEM) and X-ray diffractometer (XRD). The electrochemical properties of the NCM crystal layers were determined by galvanostatic charge-discharge tests of half-cells using both 2032 coin-type cells and pouch cells. Results and Discussions Primary single crystals with well-developed facets, whose size are 5–10 μm, were directly and densely grown from a Pt substrate. The typical optical and SEM images of the crystal layer grown from Li2MoO4 flux with 60 mol% of solute concentration is shown in Figure. The crystal layer was primary attributed to layered NCM phase crystallized into R-3m lattice, as determined by XRD profiles. Cross-sectional SEM observation revealed that small voids with submicron meter scale were formed insides of the NCM crystal layers. The electrode density was roughly estimated to be 3.7 g·cc-1. The reaction conditions such as a solute concentration strongly influenced the morphology and density of the electrodes. Their electrochemical properties of the NCM crystal layers were studied as an additive-free electrode by using coin cell system having Li metal electrode as a counter electrode. The cut-off voltage range is 2.8 to 4.3 V and the current density was controlled to be 20 μA·cm-2. The current capacity of the cell was found to be increased from 200 to 2000 μAh range in proportional to the weight of the NCM crystals, implies that the electrolyte penetrates homogeneously into the whole electrode, leading not to make critical internal resistances. The specific discharge capacity of first cycle was 96 mAh·g-1 with low coulomb efficiency of 67%. These poor capacity and large irreversible capacity were primary associated to the formation of Li2 MeO2 (Me = transition metal) phase in the balk and of any resistant layer at the electrolyte interface. In facts, the flat plateau was appeared at 1.2 V (vs. Li+/Li) under the lower limiting voltage extending to 1.0 V. Furthermore, introducing ultra-thin layer of Li4Ti5O12 homogeneously coated on the NCM electrodes significantly improved the specific discharge capacity to be 130 mAh·g-1. We also demonstrated a pouch cell test consisted of the NCM crystal layer cathode and a Si/CNT composite anode for achieving high energy density. The detail of the data on this matter will present at PRiME2016. Conclusions We proposed herein a new possibilities on electrode structure for achieving high energy LIBs. Directly grown NCM crystal layer from the current corrector by using flux growth approaches realized high energy density additive-free electrode of LIBs. Acknowledgments This research is partially supported by JST-CREST and KAKENHI (25249089). Figure 1